This invention relates to ground treatment and, more particularly, is concerned with strengthening ground of otherwise inadequate load bearing capacity by the formation therein of stone columns.
There are two well known methods for the formation of stone columns in the ground in order to provide support for buildings. These are both methods of ground improvement rather than of piling since the strength of each column is derived from lateral soil reaction around the column, and they are normally applied to cohesive ground or "fill" soils rather than to cohesionless soils since these latter soil types may in general more readily be compacted by the use of heavy vibrator equipment to increase their strength.
The most common of the known methods is to use a special vibrator, sometimes known as a Vibroflot, which expels water or air from its body as it sinks into the ground, thus forming a hole. The hole is then filled with stone and the stone is compacted into the ground in stages using the vibrator. Thus a stone column is formed in the ground which serves to strengthen it and which also provides a drainage path which is beneficial to the rapid consolidation of the ground as structural loads are subsequently applied.
The second known method is to drive a tube into the ground using a gravel plug in the base or the tube upon which acts a hammer which is raised and dropped within the body of the tube. When the tube has been driven to a desired depth it is then held in a fixed position by wire ropes, while the plug is expelled by further driving. Subsequently, a stone is placed within the tube and compacted by the hammer, the tube is withdrawn so that finally a stone column is formed in the ground. This stone column acts in the same manner as described above for the case where a vibrator is used in its insertion.
There are some disadvantages attached to both these methods in that, in wet ground conditions when using the vibrator-based method, the bore hole formed must be kept full of water which involves the displacment of significant amounts of water onto the site; while, in the case of the driven-tube methods, the driving forces needed to expel the plug are high and involve considerable forces on the equipment during the plug ejection and tube withdrawal.